BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to a method and apparatus for transmitting video data.
Description of the Prior Art
[0002] In general, when a video signal (a television signal) is transmitted via a communication
satellite, a frequency band of higher than 10 GHz is used. Radio waves of such a high
frequency band are considerably attenuated by rainfall. In transmitting a video signal
through a communication satellite, an FM modulation scheme is generally employed.
However, if the signal level of the FM signal is attenuated to become lower than a
threshold carrier-to-noise (C/N) ratio, the quality of the reproduced picture deteriorates
rapidly. Accordingly, when the video signal is transmitted through the communication
satellite, it is necessary to make up some suitable counterplan for bad weather such
as rainfall or the like. In this connection, a serviceability of higher that 99.9%
is requested in the case of bad weather.
[0003] In that case, if a direct broadcast by satellite (DBS) is performed between stationary
stations, the image quality of the picture can be prevented from being deteriorated
by increasing the transmission output. However, when the broadcasting station transmits
a video image to its own station from a spot covered via a communication satellite,
if the transmission output is increased, then not only the transmitter but also the
power supply source apparatus or the like must be enlarged in size and increased in
weight. Therefore, an increase of the transmission output is not desirable.
[0004] When, on the other hand, a video signal is digitized to perform the DBS, 4-phase
shift keying (PSK) is frequently used as the modulation system. When the 4-phase PSK
is employed, the attenuation of the signal by rainfall or the like can be avoided
by switching it to a 2-phase PSK.
[0005] According to this method, however, the signal band is widened in the 2-phase PSK
so that an expensive frequency band of the communication satellite must be increased.
This is described in an article by Y. Yasuda et al., "THEORETICAL BIT ERROR RATE PERFORMANCE
OF SOFT DECISION VITERBI DECODING,"
Transaction of the Institute of Electronics Information and Communication Engineers, No. CS80-126, pp. 31 to 36, Sept. 24, 1980.
[0006] As described above, when the video signal is transmitted via the communications satellite,
the image quality of the received picture is deteriorated by rainfall or the like.
In order to remove this disadvantage, the prior art apparatus must be made large-sized,
heavy in weight or complicated in construction. Alternatively, the signal band is
widened.
OBJECTS AND SUMMARY OF THE INVENTION
[0007] Therefore, it is an object of the present invention to provide an improved method
and apparatus for transmitting video data in which the above-mentioned defects can
be eliminated.
[0008] More specifically, it is an object of the present invention to provide a method and
apparatus for transmitting video data in which the best image quality can be obtained
for any given transmitting condition.
[0009] It is another object of the present invention to provide a method and apparatus for
transmitting video data in which the worst cases where an apparatus cannot be operated
due to bad weather such as a rainfall or the like can be avoided.
[0010] It is a further object of the present invention to provide a method and apparatus
for transmitting video data in which the apparatus can be reduced in size.
[0011] The present invention obtains these objects by a method and apparatus for transmitting
data by compressing the data, convolutionally encoding the compressed data, transmitting
the convolutionally encoded, compressed data at a predetermined speed, receiving the
convolutionally encoded, compressed data, Viterbi decoding the convolutionally encoded
compressed data, and expanding the Viterbi decoded data, wherein the combination of
the compressing ratio and the encoding ratio of the data is varied while the transmitting
speed of the data is maintained constant.
[0012] The above, and other objects, features and advantages of the present invention, will
be apparent in the following detailed description of a preferred embodiment of the
invention when read in conjunction with the accompanying drawings, in which like reference
numerals are used to identify the same or similar parts in the several views.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013]
Fig. 1 is a diagram of an example of an encoding gain, and to which reference will
be made in explaining the present invention;
Fig. 2 is a block diagram showing an outline of an embodiment of an apparatus for
transmitting video data according to the present invention; and
Fig. 3 is a table used to explain the operation of the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0014] In order to understand the present invention more clearly, let us first explain a
fundamental principle of the present invention.
[0015] Fig. 1 shows an example of an encoding gain wherein values in the table of Fig. 1
are quoted from the foregoing literature, "THEORETICAL BIT ERROR RATE PERFORMANCE
OF SOFT DECISION VITERBI DECODING." Referring to Fig. 1, "(4,3)" convolutional code,
for example, indicates that, of the code of 4 bits, 3 bits are assigned to information
bits and the remaining 1 bit is assigned to an error correcting code. More specifically,
a (n0, k0) convolutional code indicates that, of n0 bits encoded, the information
bit is formed of k0 bits and the error correcting bit is formed of remaining (n0 -
k0) bits. Further, a value Q is a soft decision value Q.
[0016] Assuming that an encoding ratio R is expressed as R = k0 / n0 then, this equality
demonstrates that, according to Fig. 1, the larger the encoding ratio becomes the
encoding gain becomes larger in the convolutional code regardless of the value Q.
This is equivalent to the fact that, if the encoding ratio R is increased, the gain
of the transmission path is equivalently increased, whereas the attenuation of the
signal (radio wave) is reduced correspondingly.
[0017] From such a standpoint, the gain of the transmission path can be equivalently changed
by the encoding ratio of the convolutional encoding and using data compression in
the DBS of the digital video signal, so that, according to the present invention,
image data is compressed and is convolutionally encoded and is transmitted. Simultaneously,
in response to the condition of the transmission path, the data compressing ratio
and the convolutional encoding ratio are varied so as not to vary the transmission
rate.
[0018] The embodiment of the present invention will now be described in detail hereinunder.
[0019] Referring to Fig. 2, there is provided a transmitting apparatus 1 which is located,
for example, at the news source place. A receiving apparatus 2 is located, for example,
at the broadcasting station and a communication satellite 3 for DBS is provided. In
this embodiment, a transmission rate at which image data is transmitted from the transmitting
apparatus 1 to the receiving apparatus 2 via the communication satellite 3 is selected
to be 24 Mbps (Megabits per second).
[0020] As shown in Fig. 2, in the transmitting apparatus 1, a digital video signal S₁ input
at terminal 11 is supplied to a data compressing circuit 12 which is what might be
called a high efficiency encoding circuit. In that case, the digital video signal
S₁ is formed according to a signal format which conforms, for example, to the CCIR
Recommendation 601. In other words, a luminance signal is converted to a digital luminance
signal by a sampling frequency of 13.5 MHz and a quantization bit number of 8 bits,
whereas red and blue color difference signals are converted to digital red and blue
color difference signals by a sampling frequency of 6.75 MHz and a quantization bit
number of 8 bits. Thus, the digital video signal S₁ is a signal of 216 Mbps on the
whole.
[0021] The data compressing circuit 12 data-compresses the signal S₁ to provide data S₂
according, for example, to an adaptive predictive coding system and a variable length
coding system. At that time, in accordance with a control signal Sc, a data amount
N after the signal S₁ is compressed and the data compressing ratio C are controlled
to fall within any one of groups I to IV on the rows 2 and 3 in Fig. 3 where an equality
of C = (P-N)/P is established. The value P in this equation represents the data amount
when the reference data compression is carried out without the application of the
present invention, and is selected to be equal to the transmission rate of 24 Mbps.
That is, if the reference data compression is contained, the original data S₁ is compressed
to the data S₂ by the compression ratio is expressed as

[0022] Further, a code generating circuit 13 generates, for example, a cyclic redundancy
check (CRC) code S₃ as an error checking code, and this CRC code S₃ is added to the
data S₂. In this embodiment, the aforementioned data amount N and compressing ratio
C is applied to the signal which includes the code S₃.
[0023] The data S₂ to which is added the code S₃ is supplied to a convolutional encoding
circuit 14. The convolutional encoding circuit 14 convolutionally encodes the data
S₂ to data S₄ and can also vary freely the code (n0, k0) of the data S₄ in the convolutional
encoding and the encoding ratio R to be any of those described in the groups I to
IV in the rows 4 and 5, respectively, of the table of Fig. 3 in accordance with a
control signal Se. In that case, if the data compression in the data compressing circuit
12 belongs to the group I, then the data compression in the encoding circuit 14 must
belong to the group I. In other words, the data compressing ratio C in the data compressing
circuit 12 and the encoding ratio R in the encoding circuit 14 are controlled so as
to lie in the same group.
[0024] Accordingly, the transmission rate of the data S₄ becomes constant, for example,
24 Mbps regardless of the groups I to IV. That is, the data compressing ratio C and
the encoding ratio R of each of the four groups I to IV are determined such that the
transmission speed becomes constant. In this embodiment, the transmission speed is
selected to be 24 Mbps so that numerals [2,3,4,8] which are factors of [24] are selected
as a denominator of a fraction which indicates the data compressing ratio C and the
encoding ratio R.
[0025] The data S₄ from the encoding circuit 14 is supplied to a transmitting circuit 15
which is comprised of a modulating circuit, a frequency converting circuit, a power
amplifier and the like not shown. In the transmitting circuit 15, the data S₄ supplied
thereto is converted, for example, to a 4-phase PSK signal S₅, and this signal S₅
is transmitted to the communication satellite 3 through an antenna 16.
[0026] The signal S₅, relayed by the communication satellite 3, is received by a so-called
parabola antenna 21 in the receiving circuit 22 of, for example, a double superheterodyne
system, in which it is demodulated to provide the data S₄.
[0027] The demodulated S₄ data is supplied to a Viterbi decoding circuit 23 and is thereby
demodulated as the data S₂ in response to the control signal Se. The data S₂ is supplied
to a data expanding circuit 24, in which it is expanded in response to the signal
Sc to provide the original video signal S₁. This original video signal S₁ is supplied
to an output terminal 25. In the foregoing, the data expanding circuit 24 is what
might be called a high efficiency decoding circuit.
[0028] In that case, the data expanding circuit 24 separates and derives the CRC code S₃,
and this CRC code S₃ is supplied to an error deciding circuit 26 which decides the
error condition or error rate of the code S₃. The decided result is supplied to a
control circuit 27 which derives the above-mentioned control signals Sc and Se in
accordance with the decided result of the deciding circuit 26. The signal Sc, which
represents a compressing ration C, is supplied to the data compressing circuit 12
and the encoding circuit 14, whereas the signal Se, which represents an encoding ratio
R, is supplied to the encoding circuit 14 and the decoding circuit 23. The circuits
12, 24 and 14, 23 are controlled by the signals Sc and Se such that, if the error
rate of the CRC code S₃ becomes larger, a group of a lower number is selected from
the groups I to IV. More specifically, when the transmission condition is good, the
group IV is selected. When the transmission condition becomes worse, the groups III,
II and I are selected in that order.
[0029] Although the method for supplying the control signals Sc and Se to the data compressing
circuit 12 and the encoding circuit 14 from the receiving apparatus 2 is not specific,
the control signals Sc and Se may be transmitted to the data compressing circuit 12
and the encoding circuit 14 via a communication network line between the television
broadcast station and the news source place. Alternatively, the groups I to IV are
selected by the control signals Sc and Se so that the groups I to IV selected in accordance
with the control signals Sc and Se are transmitted to the transmitting side via a
communication telephone to thereby determine the compressing ratio C and the encoding
ratio R.
[0030] According to the above-described arrangement, when the weather condition is good,
the video signal S₁ is transmitted by the group IV. At that time, the data compressing
ratio C for the video signal S₁ is as small as 1/8 so that a video signal S₁ of good
image quality can be transmitted as shown in the row 7 of Fig. 3.
[0031] When the weather condition is bad, the video signal S₁ is transmitted by the group
I. At that time, the encoding ratio R for the video signal S₁ is small, i.e., the
encoding gain is large as shown in the row 6 of Fig. 3 so that although the image
quality is deteriorated because the data compressing ratio C is large, the video signal
S₁ can be transmitted at any rate. Therefore, it is possible to avoid the worst cases
where the transmitting apparatus cannot be operated at all. If the weather condition
becomes fine, the group is changed from the group I to the group II or the group III
or IV is selected, then the image quality becomes better.
[0032] As described above, according to the present invention, since the video signal is
data-compressed and convolutionally encoded and then transmitted with the various
combination of the data compressing ratio C and the encoding ratio R in response to
the transmission conditions at that time, the best image quality can be obtained under
the above-mentioned transmission conditions and the worst cases, where the apparatus
cannot be operated by the rainfall or the like, can be avoided.
[0033] Further, since the encoding ratio C and the data compressing ratio R are combined
to form a group and the transmitting speed is selected to be constant, for example,
24 Mbps in any of the groups, the characteristics of the apparatus from the transmitting
circuit 15 to the receiving circuit 22 need not be varied in response to the transmitting
conditions and can be commonly used under all transmission conditions. Therefore,
the scale of the apparatus can be reduced, and particularly in the transmitting apparatus
1, not only the transmitting circuit 15 but also a power supply source apparatus (not
shown) can be made compact in size and made light in weight. Thus, the transmitting
apparatus 1 and the power supply source can be transported or installed at a news
source spot with ease.
[0034] Having described a preferred embodiment of the invention with reference to the accompanying
drawings, it is to be understood that the invention is not limited to that precise
embodiment of the invention and that various changes and modifications could be effected
by one skilled in the art without departing from the spirit or scope of the novel
concepts of the invention as defined in the appended claims.
1. A method of transmitting data comprising the steps of
(a) compressing (12) the data;
(b) convolutionally encoding (14) the compressed data;
(c) transmitting (15,16) the convolutionally encoded, compressed data at a pre-determined
speed;
(d) receiving (21,22) the convolutionally encoded, compressed data;
(e) Viterbi decoding (23) the convolutionally encoded, compressed data; and
(f) expanding (24) the Viterbi decoded data, wherein the combination of the compressing
ratio and the encoding ratio of the data is varied while the transmitting speed of
the data is maintained constant.
2. The method according to claim 1, further comprising the steps of detecting the
error situation in the expanded Viterbi decoded data, and changing the combination
of the compressing ratio and the encoding ratio of the data according to the detected
error situation.
3. The method according to claim 1 or 2, wherein the transmitting step comprises transmitting
the data via a satellite (3).
4. The method according to anyone of claims 1 to 3, wherein the combination of the
compressing ratio and the encoding ratio of the data is changed according to the weather.
5. An apparatus for transmitting data comprising:
(1) compressing means (12) for compressing data according to a selected ratio;
(2) encoding means (14) for convolutionally encoding the compressed data according
to a selected ratio;
(3) transmitting means (15,16) for transmitting the convolutionally encoded, compressed
data at a pre-determined speed;
(4) receiving means (21,22) for receiving the convolutionally encoded, compressed
data;
(5) decoding means (23) for Viterbi decoding the convolutionally encoded, compressed
data;
(6) expanding means (24) for expanding the Viterbi decoded data; and
(7) wherein the compressing means (12) and the encoding means (14) vary the combination
of the compressing ratio and the encoding ratio of the data while the transmitting
means (15;16) maintains the transmitting speed of the data constant.
6. The apparatus according to claim 5, further comprising means (26,27) for detecting
the error situation in the expanded Viterbi decoded data and controlling the compressing
means (12) and the encoding means (14) to change the combination of the compressing
ratio and the encoding ratio according to the detected error situation.
7. The apparatus according to claim 5 or 6 , wherein transmitting means comprises
a satellite data transmission network (15,16,3).
8. The apparatus according to claim 5, further comprising means (26,27) for controlling
the compressing means (12) and the encoding means (14) to change the combination of
the compressing ratio and the encoding ratio to compensate for changes in the weather
situation.